This commit introduce a new sink: syslog_sink.
This sink is Linux only, and will write log entries to the system
logger, using the syslog() library call.
It is instanciable using spdlog::syslog_logger(name). Note that
the suffix _st or _mt is not present, as syslog() is thread-safe.
I also applied @gabime reviews and added license header.
This commit introduces 3 new log level. Thoses are:
+ NOTICE, which is a bit worse that INFO, but still not a warn.
+ ALERT, for case worse that critical.
+ EMERG, application is unusable.
With those 3 log levels, spdlog now has all log level accepted by
the syslog() system call.